Millimeter wave Cassegrain-type antennas have many applications. In particular, these antennas are used in tracking radar systems for guided or seeker missiles. Such antennae tend to have quite narrow beam widths in comparison to L or X band radars. For example, a 30 cm diameter antenna operated at 94 GHz (.lambda.=3.2 mm) typically has a beam width of 0.7.degree. as compared to 7.degree. for an X band antenna of equal diameter. Such a narrow beam width is desirable from the standpoint of electronic counter measures (ECM) and power requirements, since relatively low power is required for long range sensing and the antenna signal is relatively immune to jamming.
However, while conventional Cassegrain-type antennas work well for long ranges, at close range (less than three miles), the narrow beam width of the antenna signal illuminates only a small part of a large target, such as a ship. For example, with a 0.7.degree. beam, only a 12 m diameter patch is illuminated at a distance of 1 km. Since a target like an aircraft carrier may be 300 m long, the aimpoint of a monopulse seeker system tends to wander and will lock on the bow or stern of a ship unless a strong and stable reflector is found at an intermediate location. This is not desirable in terms of the effectiveness of a missile in destroying targets. Because a target like an aircraft carrier is much larger in azimuth than elevation, it would be advantageous to have a radar beam width which is much narrower in azimuth than elevation.
Several methods have been developed for selectively varying the beam width of antennas. For example, U.S. Pat. No. 3,866,233 to Schmidt discloses Cassegrain and Gregorian-configured dish antennas having a switchable beam width wherein a subreflector is effectively reduced in size by moving an annular outer portion out of focus position. The '233 patent antenna permits a narrow beam width antenna to be switched to a wide beam width configuration, initially, in order to acquire a transmitting station it is seeking. Then, once the station has been acquired, the antenna is switched back to its narrow beam width configuration for normal operation. This antenna is useful for a satellite communications system wherein a antenna must track a small object which is always at the same long distance range away from the antenna. Schmidt's antenna also reduces the entire radius of the sub-reflector or reflector which in turn widens both the elevation (vertical) and the azimuth (horizontal) characteristics of the antenna. This is not desirable for a missile because the missile needs to retain the azimuth pattern in order to effectively track a target. Also, the Schmidt antenna would be too bulky to be used in a missile system.
Other examples include U.S. Pat. No. 3,938,162 to Schmidt, which discloses a variable beam width antenna in which the azimuth and elevation beam widths are independently adjustable by cylinder-parabol reflectors at right angles which can be telescoped; U.S. Pat. No. 2,408,373, which discloses an antenna having a main reflector with hinged sections that can be tilted to adjust the beam width; U.S. Pat. No. 3,254,342 to Miller, which discloses a variable beam width Cassegrain antenna having a subreflector made of an elastic material, the shape of which is varied to control the aperture area; and U.S. Pat. Nos. 4,253,100 to Commault et al. and 4,612,550 to Brucker et al., which disclose an inverse Cassegrain antenna having a hinged subreflector for changing the effective area of the subreflector. These prior art variable beam width antennas are either too bulky for use in missile systems; vary both the azimuth and elevation beam width, which is inappropriate for missile tracking radar systems; or require the use of mechanically moveable parts, which at the millimeter wavelength range requires extremely high precision. Such mechanical systems are further sensitive to the substantial shock and vibration conditions to which missile systems are subjected. Subreflectors which have displacable parts are also subject to spurious reflections from any of the displaced parts.